Optical telecommunication systems require a variety of devices for generating, transmitting, amplifying, filtering, switching, detecting and otherwise processing optical signals. Certain of these devices are integrated optical components that utilize a number of optical waveguides formed on a single substrate so that optical signals traveling in the respective waveguides can be locally processed. Like integrated electrical circuits, there is constant pressure to increase the degree of integration of integrated optical devices and components so they can be made smaller and less expensive to produce and operate.
One example of an integrated optical device (apparatus) used in telecommunications for optical switching applications is an integrated optical Mach-Zender interferometer.
FIG. 1 is a plan view of a section of a prior-art Mach-Zender interferometer 10 thermal optical switch apparatus having two separated waveguides, with a heating element over one of the waveguides to perform thermal optical switching. Waveguides 16 and 20 represent the two arms of the interferometer and have respective output ends (“ports”) 22 and 24. The waveguides are formed on a substrate 26. Waveguide 16 supports a lightwave 30, and waveguide 20 supports a lightwave 32. Interferometer 10 includes a central section 40 where the waveguides are parallel and separated by a distance D1 such that no coupling between the waveguides occurs. Surrounding central section 40 are first and second coupling sections 42 and 44 in which coupling between the waveguides occurs.
Atop waveguide 16 in central section 40 is a heating element 50 that selectively heats a portion of length L of the waveguide. The change in temperature ΔT of waveguide 16 in section 40 over the select portion results in a change in refractive index ΔN of the material making up the waveguide over that portion. This refractive index change in the waveguide over a select distance translates into a difference in the phase between the interferometer arms.
When the phase difference between the interferometer arms is zero, lightwave 30 initially inputted into waveguide 16 upstream of first coupling section 42 is transferred entirely to waveguide 20 in coupling sections 42 and 44 to form lightwave 32, which is outputted at output port 24. However, the phase difference between the interferometer arms can be altered through heating via heating element 50. The change in phase difference changes the amount of light coupled from waveguide 16 to waveguide 20 at second coupling section 44 and thus the balance of light outputted at output ports 22 and 24. This is the basis for a thermal optical switch (TOS).
In interferometer 10, the distance D1 must be great enough to prevent heating of waveguide 20 when waveguide 16 is heated. If waveguide 20 is not properly thermally isolated from waveguide 16, it too will experience a change in temperature, which will reduce the temperature differential and thus the phase differential between the waveguides. This in turn diminishes switching performance. Unfortunately, providing the needed thermal isolation by spacing the waveguides farther apart takes up valuable substrate space and reduces the degree of integration of the apparatus.
Certain prior art interferometers include an air-filled trench between waveguides 16 and 20 to provide thermal insulation, which allows the waveguides to be placed closer together. However, there is still pressure to increase the degree of integration and reduce the amount of power needed to operate the apparatus. This is due in part to the fact that tens to hundreds of interferometer apparatus like apparatus 10 may be formed on a single substrate, with each apparatus requiring about 0.5 W to operate. This results in a substantial thermal budget for a given substrate. It is therefore desirable to further increase the level of integration of interferometer-based thermal switch apparatus while also maintaining or decreasing their thermal energy budget.